Selective catalytic reduction (SCR) systems may be used in vehicles to reduce NOx emissions. SCR systems involve injection of a reductant upstream of an SCR catalyst. The reductant, or reductant products, then reacts with exhaust NOx species to create byproducts such as nitrogen and water. The system may also include mixers to facilitate mixing of the injected reductant with exhaust gases before interacting with the SCR catalyst.
Various approaches may be used to control emission systems including SCR catalysts to enable good mixing of reductant and exhaust, and minimize reductant slip from the catalyst. One example approach is illustrated by Kimura et al. in JP 2008-128046A. Therein, a reductant injector is provided downstream of a turbocharger turbine and upstream of the SCR catalyst. Specifically, reductant is injected into an exhaust flow coming out of the turbocharger.
However, the inventors herein have recognized several potential issues with such an approach. As one example, reductant vaporization may be significantly degraded due to injection into exhaust gases downstream of the turbocharger. In other words, significant exhaust gas heat may be extracted by the turbine thus reducing the initial vaporization upon injection of the liquid reductant into the exhaust. Such reduction in vaporization may reduce NOx conversion efficiency of the SCR catalyst, as well as result in increased reductant deposits.
In one example, the above mentioned issues may be addressed by a method of operating an engine including an emission control system, the emission control system comprising a catalyst downstream of an exhaust turbine of a boosting device, the emission control system further comprising a reductant injector upstream of the turbine. In one embodiment, the method comprises, injecting reductant to the exhaust upstream of the turbine, mixing the injected reductant with exhaust gas via the turbine; and delivering the mixed reductant to the catalyst.
In this way, it is possible to not only improve mixing by utilizing action of the turbine, but further it is possible to increase vaporization by injecting reductant upstream of the turbine before exhaust gas temperature drops due to the energy extracted by the turbine. Specifically, by injecting reductant upstream of an exhaust turbine, and by flowing the reductant through the turbine vanes and blades, a finer atomization of the reductant may be achieved. Additionally, the turbulence generated in the turbine may improve mixing of the reductant with the exhaust.
Furthermore, by injecting the reductant upstream of the turbine, some of the improved atomization and mixing benefits may be synergistically enhanced by increased vaporization due to the higher exhaust gas temperature upstream of the turbine as compared with downstream of the turbine. Yet, at the same time, by positioning the SCR catalyst downstream of the turbine, an appropriate temperature regime for the SCR catalyst can still be maintained.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.